Project Summary Gene therapy using autologous CD34+ cells is a promising treatment for primary immunodeficiency, particularly for individuals without optimal allogeneic donors. SCID-X1 is caused by mutations in IL2RG, which encodes the common gamma chain (?c) of multiple cytokine receptors. Boys with SCID-X1 lack T and NK cells, and their B cells fail to produce antibodies due to the lack of IL-7, IL-15 and IL-21 function respectively. This project seeks to test the efficacy and safety of a new self-inactivating lentiviral (LV) vector to treat SCID- X1. We hypothesize that this trial will improve immune reconstitution through the introduction of low dose busulfan conditioning (Aim 1) and improve safety through the change from a gammaretroviral (?RV) vector used in previous trials to the LV vector in this trial (Aim 2). Previous trials of gene therapy for SCID-X1 have infused cells without chemotherapy conditioning, which resulted in robust T cell recovery and gene marking, but negligible gene marking in B cells and failure of humoral immune reconstitution. Initial development and marking in NK cells was not sustained. In Aim 1 we will examine the impact of low dose busulfan conditioning on 1) cell type specific engraftment and gene marking, 2) in vivo T cell reconstitution, T cell phenotype and TRB repertoire by deep sequencing, 3) in vivo humoral immune reconstitution, B cell number, phenotype, IL-21 dependent function and IGH repertoire by deep sequencing, 4) NK cell number, phenotype and function. Previous trials of gene therapy for SCID-X1 have used a ?RV vector with intact viral promoters/enhancers, which resulted in 5/20 patients developing T cell leukemia due to insertional oncogenesis. Gene therapy using a self-inactivating ?RV vector in which viral enhancers have been deleted shows encouraging evidence of reduced insertion sites near lymphoid oncogenes, but an initial insertion site pattern that is still risky. The proposed trial in this application will further improve safety by using a self-inactivating LV vector. In Aim 2 we will investigate the initial insertion site pattern in the patients? CD34+ transduced cells and compare samples from the proposed trial to historical trials using ?RV, analyze insertion site profile in peripheral blood after gene therapy to perform lineage tracing and compare clustering with samples from previous trials.